want to make a special type speaker

[kartik nighania]

im a newbie.. so sorry for non technical terms I am going to use..
I had made a circuit that gets opened for a very short time and then gets closed again as it was before. (this happens in less than half second)
but I want to make an alarm that will run when the circuit gets opened and will continue to ring even after the circuit is closed...
actually I had made a circuit using an LDR light sensor when the circuit gets opened and closed I want an alarm to work for minimum of 15 sec...
PLS HELP ITS FOR MY SCIENCE EXHIBITION.......

 

[BradtheRad]

Re: want to make a SPECIAL TYPE SPEAKER

A 555 timer IC might do the job. It's one of the most popular and easy-to-use chips.

You would arrange it to trigger when it detects a drop in volt level.
The output goes high for 15 seconds, whether the switch is closed again quickly, or remains open.
The length of time delay is set by the 150 kOhm resistor and 100 uF capacitor (although the values are flexible).

Screenshot with scope traces:



There is also a way to trigger it from your LDR, if you wish.